1. Field of the Invention
The present invention relates to air-cooled heat exchangers generally comprising a battery of heat exchanger units and more particularly to a novel shield to prevent convective heat from reaching personnel working within one of a battery of heat exchanger units that has been temporarily taken out of service for maintenance or repair during operation of the remaining units.
2. Description of the Prior Art
Air-cooled heat exchangers are widely used in the energy, chemical and manufacturing industries, and are generally referred to as direct system heat exchangers since they transfer heat from a fluid directly to ambient air.
Each air-cooled heat exchanger unit consists of a bundle of horizontally oriented tubes connected in parallel between fluid inlet and outlet headers. The outside of the tubes is formed with fins thereby increasing the air side surface so as to compensate for the low heat transfer coefficient of the cooling air. Each unit is equipped with at least one air-moving device which is normally an axial flow, propeller type fan that is located either above or below the tube bundle and causes the cooling air to flow generally upward and across the tube bundle in a direction that is approximately perpendicular to the orientation of the finned tubes. Fans that are located above the tube bundle pull the cooling air across the tube bundle and are referred to as induced draft fans. Fans that are located below the tube bundle push the cooling air across the tube bundle and are referred to as forced draft fans.
In air-cooled heat exchangers of this general type, and particularly where the fluid flowing through the tubes is steam, it is necessary to continually remove non-condensable gases from the outlet headers of the bundles. Otherwise, these gases will collect and form stagnant pockets in bundle tubes and headers which will freeze condensate in the winter and cause inefficient operation during the summer by blanketing heat transfer surfaces. Conventionally, non-condensable gases are so removed through vent condensers, dephlegmaters, or vent tubes connecting the bundle outlet headers with a common manifold generally leading to the first stage of a steam jet ejector or other suitable equipment.
During low steam load conditions and/or cold weather, the operator needs to reduce the quantity of cooling air through the bundles of the air-cooled heat exchanger. However, if this were to be done by merely shutting off certain fan motors while leaving others on, the resulting differences in steam flow rates pressure drops cause a dangerous and damaging condition in which the tubes of the bundles serviced by the operating fans would fill with non-condensable gases. To circumvent this, control procedures are recommended by the manufacturer for cyclically turning some fans on and others off as recited in a predetermined operating regimen of about 15 minutes duration for each cycle. The fan cycling is intended to scavenge the non-condensable gases from those tubes that have accumulated these gases while allowing the bundles serviced by the operating fans to fill with non-condensable gases once more. However, since all headers of conventional air-cooled heat exchangers of this type connect to a common manifold, these cyclic controls inherently interfere with operation of the system for removing non-condensable gases.
Some plant operators do not like to rely on a cyclic control system of this type because of its uncertainty, and hence it is the more common practice to place more reliance on equipment especially installed for controlling the amount of cooling air passing over the tubes of each bundle, such as by means of louvers, multi-speed fan motors, variable speed fan drives, variable pitch fan blades, or combinations of them. However, this added reliance increases the frequency of use of such equipment with a concomitant increase in the frequency of maintenance and repair of the equipment, most if not all of which is located within the air-cooled heat exchanger unit, thus requiring that maintenance personnel work within the unit and that the fan be taken out of service.
In air-cooled heat exchanger systems comprising a battery of air-cooled heat exchanger units arranged in side-by-side fashion with the tube bundles running parallel to one another, the cooling air flowing over the tube bundle absorbs the heat from the fluid flowing through the finned tubes and may be heated to temperatures of 200° F. or higher. Heat exchanger units equipped with induced draft fans are generally limited to applications where the temperature of the air exiting the tube bundle does not exceed 220° F. so as not to damage the fan blades, bearings, or other mechanical equipment located in the path of the heated air. Heat exchanger units equipped with forced draft fans are generally recommended where the air temperature exiting the tube bundle exceeds the limit set for units equipped with induced draft fans and particularly where such exiting air temperatures may exceed 350° F. due to low air flow operation or the shut down of the fan. The cooling air will normally enter from beneath the heat exchanger unit and exit, as heated air, from the top of the unit. This airflow pattern results in a lower air pressure area directly under the heat exchanger and a higher pressure area directly above it. Thus, whenever the fan is shut down in a heat exchanger unit that is being taken out of service for maintenance or repair on the fan or other mechanical equipment located within the unit, this differential in air pressure will cause some of the heated air exiting from the adjacent operating heat exchanger units to be drawn over and across the tube bundle of the unit which has been taken out of service. Furthermore, even though the fan has been shut down, it is desirable and normal practice to keep the hot fluid flowing through the finned tubes of the bundle located in the unit that has been taken out of service. However, such practice adds further heat to the already heated air that is being drawn down from the adjacent operating heat exchanger units. This heated air creates a very harsh environment for maintenance personnel if they should have to work within the heat exchanger unit that has been taken out of service.
Heretofore, relief for maintenance personnel required to work within an out of service heat exchanger unit has been in the form of portable fans which blow fresh air into the work area, and plywood boards placed on top of the tube bundle in units equipped with induced draft fans, and on top of the fan guard in units equipped with forced draft fans, in an attempt to block the heat generated by the hot fluid flowing through the tubes of the bundle and the heated air being drawn from the adjacent operating heat exchanger units. This arrangement has proven less than satisfactory since the plywood boards manage, at best, only to deflect rather than to prevent the heated air from being drawn into the work area.